Wireless charging is an evolving technology that may bring a new level of convenience of charging millions of electronic devices. In a wireless charging system, in particular, in an inductive wireless charging system, energy is transferred from a power transmitter, through magnetic coils coupled to the power transmitter, to one or multiple power receivers. Charging efficiency, which is the ratio of received power over transmitted power, is the most critical parameter in wireless charging. Improving the charging efficiency will effectively reduce the energy wasted during transmission, prevent heat discharges or over-voltage damages of the charging and to-be-charged devices. Also the improved efficiency could eliminate unnecessary electromagnetic radiations, avoiding potential influences on human's health.
There are mainly two approaches for efficiency optimization: (1) hardware approach, by using high efficiency components such as integrated circuits (ICs); (2) software approach, by tuning input power based on an optimization method. Most of the Qi protocol based wireless-charging systems in today's market rely on the hardware approach, and only tune to achieve a stable output voltage at the receiver rectifier. These systems use a power transmission pad and a compatible receiver in a portable device. In operation, a to-be-charged device is placed on top of the power transmission pad, which charges it via resonant inductive coupling. The hardware approach is greatly limited by cost, product design, and IC technology. On software approach, wireless charging systems based on AirFuel protocol uses Bluetooth Low Energy (BLE) to provide feedback information from a power receiver to a transmitter, so power transmitter can calculate a current efficiency and tune the input power when receiving the feedback packet. However, the BLE feedback packet interval is large due to the low energy characteristics, and the tuning step is small because of the inconsistency of receiver (RX) load condition under different input power, so this mechanism takes a long time to converge to good efficiency.
Therefore, it is desirable to have a wireless charging system with optimal charging efficiency, fast convergence time, and reliable output power.